The invention relates to a method for the production of a strip-like metallic composite material by the high-temperature dip coating of a metallic carrier strip, onto the surface of which a thin layer of a melted metallic depositing material is crystallized by solidifIcation when the carrier strip is being led through this depositing material, the latter being different from the material of the carrier strip, to a device for carrying out the method and to a product produced by this method.
EP 0 467 749 B1 discloses a method for the dip coating of a strip consisting of ferritic stainless steel with aluminum, in which the strip is heated in a nonoxidizing atmosphere in various steps at different temperatures, until the strip is finally dipped into a coating bath.
EP 0 397 952 B1 discloses a method for the continuous hot dip coating of stainless steel strip with aluminum, in which the strip is led, in an argon-scavenged housing, past a row of magnetic current devices, the strip is cleaned by argon plasmatron discharge, while at the same time being heated to the temperature required for dip coating, and the cleaned strip is dipped into a bath of melted aluminum.
In both methods mentioned above, therefore, aluminum is bonded to steel.
Moreover, DE 195 45 259 A1 discloses a method and a device for the production of thin metal strands, in which a metal strip is led vertically through a steel melt and at the same time has crystallized on it a layer thickness of 20-2% of the initial metal strip. Depending on thickness, the metal strip is preheated to a temperature between room temperature and a maximum of 900xc2x0 C. By means of this method, composite metal sheets are produced, in which one of the materials used is a stainless steel or an austenitic or ferritic steel.
More detailed investigations show that the expected reliably reproducible results of the bond between parent sheet and coating are not achieved in the methods disclosed hitherto.
The object of the present invention is to provide a method, a device and a product, in which an Intimate fault-free bond of the individual layers of the composite material consisting of different materials is obtained by simple means.
The above stated object is obtained by the present invention. In the invention a carrier strip is preheated on its surface before being led through the melted depositing material and is pretreated by the addition or incorporation of chemical elements, in such a way that, while the carrier strip is being led through the depositing material, a binding region consisting of a gradient material is obtained by means of diffusion actions between the pretreated surface of the carrier strip and the crust crystallizing on the surface of the carrier strip, the liquidus temperature of the gradient material being, at least in parts of this bending region, below the liquidus temperature of the carrier strip material and of the depositing material.
After the carrier strip has been dipped into the metal melt, a chill crust, which at first still has no bond with the carrier strip, solidifies on the surface within a very short time due to the subcooling which is established and the good foreign nuclei conditions. As a result of diffusion actions during and after the dipping operation, concentration profiles are established in he binding region between the crust and carrier strip and give rise to local alloying with a defined chemical composition. A gradient material with a changing chemical composition occurs along the binding region itself. The local concentrations cause a lowering of the liquidus temperature (calculation according to Wensel and Roeser) which, in parts of the binding region, is below the liquidus temperature of both the carrier strip and the depositing material. The drop in the liquidus temperature is usually accompanied by an even greater decrease in the solidus temperature. It is thus possible for liquid phase fractions to be present in the binding region, even though the carrier strip material and depositing material are in the solid state of aggregation. The liquid phase fractions ensure welding between the basic material and depositing material.
In one embodiment of the invention, a strip-like metallic composite material, comprising: a stainless steel strip; a markedly thinner layer of metallic depositing material crystallized onto the strip wherein the coating on at least one side of the strip has a thickness (dA) of dA=(0.01 to 0.3)xc3x97D wherein D is the thickness of the steel strip and, wherein a binding layer has a thickness (dB) of dB=5 to 150 xcexcm, and wherein the binding layer has a toothed line which additionally bonds positively the binding layer to be assigned both to the parent strand and to the depositing layer, and wherein there is a continuous transfer of alloying elements between the steel strip and the coating material.
The actions described above are illustrated in FIG. 4. The profile of the local liquidus and solidus temperatures is illustrated diagrammatically there in a graph in which the temperature is plotted against the space coordinate. The profile of the temperatures Tliq and Tsol makes clear the existence of liquid phase in the binding region, the basic material (carrier strip) and depositing material being in the solid state of aggregation.
It is necessary to ensure, for the purpose of reliable bonding, that the carrier strip is not too cold, and, on the other hand, the temperature cannot be selected so high that the carrier strip is melted down in the melting bath or loses strength to an extent such that it tears during transport. It was found, surprisingly, that the core of the carrier strip can be kept at an appropriate temperature, while, for the desired intimate bond, the liquidus temperature can be lowered, at least on the strip surface, in order thereby to allow diffusion-supported intermixing in the liquid state.
In advantageous developments, the invention shows various means which assist diffusion alloying in the binding region. These means may already be incorporated in the carrier strip, but they may also be applied, in assistance or alone, to the strip from outside, in that, according to the invention, the carrier strip, for the preparation of its surface, is led through a medium which contains the corresponding chemical elements penetrating at least partially into the surface.
At the same time, according to one aspect of the invention, the medium may be a gas, such as nitrogen, hydrogen, carbon monoxide, ammonia or carbon dioxide, or, according to another embodiment of the invention, a liquid, such as sulfuric acid, liquid ammonia or liquid nitrogen. It is also possible for the medium to be a solid, such as a cyanogen salt, carbonate or potassium ferrocyanide.
According to an advantageous embodiment of the invention, the carrier strips consists of steel which has a carbon content  greater than 20 ppm or a nitrogen content  greater than 20 ppm in the region of its surface.
Advantageously, the transport speed of the carrier strip and/or its penetration depth or penetration length into the liquid depositing material is set in such a way that a minimum dipping time of 50 msec is maintained, an upper limit being placed on the total dipping time by the desired layer thickness and the risk, already described above, of the carrier strip being melted down.
According to a further embodiment of the method of the invention, the surface of the carrier strip is roughened before penetration into the liquid depositing material.
In a preferred embodiment of the method of the invention the carrier strip is a carbon-containing steel which is preheated to a temperature of Tpre greater than 900xc2x0 C. at least on its surface. The depositing material is most advantageously a high-alloyed steel, in particular a chromium-alloyed steel.
The apparatus for the production of a strip-like metallic composite material by the method of the invention, consists of a metallurgic vessel for receiving the liquid depositing material, through which the carrier strip can be led in a preferably vertical run-through direction by means of pairs of rollers arranged on the entry and the exit side, and of a preheating device for the carrier strip. The preheating device is located upstream of the metallurgic vessel. According to the invention, the preheating device is arranged in a housing which is arranged in the entry region upstream of the metallurgic vessel and surrounds the carrier strip. The medium coming from a media supply is capable of being introduced via at least one feed entering the housing.
The form of the vessel through which the carrier strip is led may be selected as desired. Suitable apparatus includes dip tanks with deflecting rollers or containers with a bottom passage for the carrier strip. In the latter, the carrier strip is led vertically through the casting container. Such containers have an advantage inasmuch as, here, the penetration length and strip speed, as parameters, can be maintained as a function of the strip temperature with a high degree of reliability, since the bath height in the vessel can be set in a particularly simple and operationally expedient way.
In a particularly simple and compact apparatus for the production of a metal strand of a composite material by the method of the invention, the carrier strip is introduced into the melting bath directly out of a nonoxidizing environment. This operation may be performed by means of a housing which projects partially into the melt or, in the case of a vessel with a bottom orifice, may be performed by means of direct mounting underneath the bottom of the vessel.
When a carrier strip having the alloying agents already contained in it is used, a preheating device and a media feed, by means of which gas, preferably inert gas, is led into the housing interior, are sufficient.
Insofar as, additionally, solid or liquid media or else solely gaseous media, such as nitrogen, hydrogen, carbon monoxide or carbon dioxide, are applied to the surface of the carrier strip, either the feed is provided with blow nozzles, by means of which the gaseous medium can be injected into the interior of the housing and/or onto the surface of the carrier strip, or the feed is provided with spray nozzles, by means of which the liquid medium, for example sulfuric acid, liquid ammonia or liquid nitrogen, can be sprayed onto the surface of the carrier strip.
However, solids or pourable materials may also be used for lowering the liquidus temperature of the carrier strip, such as cyanogen salt, carbonate or potassium ferrocyanide. When pourable materials are used, these are introduced via a feed and are brought into contact with the surface of the carrier strip, and the strip, when being led past the runners, entrains the solid.
According to the invention, the medium may also take the form of a rechargeable solid body and be pressed against the surface of the carrier strip. The solids are shaped, for example, as a block which is pressed under appropriate pressure against the surface of the carrier strip.
In a further advantageous embodiment, measuring elements are used for detecting the melt temperature and the temperature and speed of the carrier strip. The measuring elements control via a processor at least one actuator for setting the speed of the carrier strip.
Furthermore, the bath height is also detected and is likewise supplied to a computer for processing. A highly accurate and reliable bath height setting can be achieved, for example by means of a vacuum container.
The carrier strip can be set, in particular by controlling the content of alloying elements, such as C, or else other alloying elements at grain boundaries, such as N, in such a way that local lowerings of the liquidus temperature occur, with the result that the binding layer has a tooth-like bonding line. This toothed line reinforces positively the intimate metallic bond which is already present.
The various features of novelty which characterize the invention are pointed out with particularity in the claims appended to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects obtained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.